Mountaineering: Freedom of the Hills is a book too few people who venture into the out-of-doors read.

In my 5th edition, there is a section in the chapter on Wilderness Travel/Walking called "Etiquette." Number six in a list of seven rules states:

"Step aside on the way downhill to let a party of uphill hikers continue forward without breaking their pace."

I've always followed that. In practice, more often than not--as Harvey says--an uphill-going person is happy for an excuse to take a breather, and will step aside first. The point is, make the gesture and then let the uphill person decide.